1. Field of the Invention
This invention relates to a piezoelectric type ink-jet printing head which can advantageously be used in an office automation machine such as a copying machine, a facsimile, a computer, a wordprocessor, a combination of these machines, or the like.
2. Description of the Related Art
An ink-jet printer is a type of printer in which liquid ink is injected through the air as small drop, a liquid column or a mist and onto a recording paper to print thereon letters, graphics, images or the like. In general, there are two types of printing heads used in such an ink-jet printer, i.e., a bubble type printing head in which bubbles are generated in the ink pressure chamber by a heater and the ink is injected by the bubble pressure, and an oscillation plate type printing head in which a pressure plate is provided at the bottom of the pressure chamber and the pressure plate is pushed by a piezoelectric actuator to inject the ink from the nozzle.
These two types of ink-jet printer heads have the following problems. In the bubble type printing head, the efficiency of the head substantially depends on the nature or the characteristics of the ink and, therefore, there are limitations in printing speed and quality. Therefore, it would be relatively difficult to satisfy the recent requirements of speed and image quality in printers using a bubble type printing head. In the oscillation plate type printing head, a pressure chamber and a nozzle are located on the upper side of a pressure plate and a piezoelectric actuator is located on the lower side of the pressure plate, so that a pressure chamber is defined between the pressure plate and the piezoelectric actuator. Therefore, the printing head is relatively complicated in construction and becomes rather expensive.
To solve the above-mentioned problems in the oscillation plate type printing head, in another type of known ink-jet printing head a part of the pressure chamber is constituted by an electrode of the piezoelectric element by itself, so that the pressure chamber is compressed by the displacement of the piezoelectric element to inject the ink in the pressure chamber through the nozzles. In this respect, the following types of ink-jet printers have been proposed in the prior art.
Japanese Unexamined Patent Publication (Kokai) No. 4-64448 discloses an ink-jet recording apparatus in which a piezoelectric element is provided with several slits to make respective actuators in the projected portions divided by these slits and parallel passages are formed in conformity with the respective projected portions, so that a displacement of the actuator in a direction perpendicular to the parallel passages can be obtained. Thus, ink in a common chamber connected to one end of each parallel passage can be injected from nozzles provided at the other end of each end parallel passage. The piezoelectric element comprises non-active areas where there are no electrodes and the thickness thereof is not changed and active areas where there are electrodes and the thickness thereof is changed. These active areas are located so as to define at least one wall of the parallel passages. The slits are filled with any filler.
However, in this ink-jet recording apparatus, since the individual actuators are divided by means of a plurality of slits in the piezoelectric element, the construction thereof is rather complicated and therefore expensive.
Japanese Unexamined Patent Publication (Kokai) No. 3-272855 discloses an ink-jet printing head comprising a base substrate, a common electrode formed thereon a piezoelectric material formed on the substrate, and a local electrodes formed on the piezoelectric material. A voltage is applied across these electrodes at the resonant frequency, in the thickness direction, of the piezoelectric material to effectively compress the ink. The pressure chamber is not divided but is common to all of the nozzles.
In this ink-jet printing head, since the pressure chamber is not divided but is commonly used for the plurality of nozzles, the structure of the pressure chamber can be simplified. On the other hand, it is difficult to keep constant the amount of ink injected from the individual nozzles and, therefore, the printing quality is reduced. Also, the direction of the ink-jet is restricted to the direction of displacement of the piezoelectric material and, therefore, it is difficult to constitute an "edge shoot" type printing head in which ink is injected in a direction perpendicular to the displacement of the piezoelectric material.
Japanese Unexamined Patent Publication (Kokai) No. 6-188475 discloses a laminated piezoelectric element in which piezoelectric material and electrodes (common and local electrodes) are alternately laminated, so that an electric field can be selectively generated in the laminated direction. A voltage is applied to the laminated piezoelectric element to exert a pressure in the pressure chamber to inject the ink from the nozzle.
However, when a pressure is exerted in the pressure chamber by the laminated piezoelectric element, since the piezoelectric element is buried in the recess of the base substrate and the walls of the pressure chamber (at least a part thereof) are not defined by the piezoelectric element, pressure is indirectly exerted to the ink in the pressure chamber by means of a membrane. Therefore, a large pressure cannot be obtained.
Japanese Unexamined Patent Publication (Kokai) No. 4-341853 discloses a piezoelectric-type ink-jet printer head comprising a laminated piezoelectric element having a plurality of divided inner electrodes and local displacement portions which are locally displaced when a voltage is applied to the electrodes. The thickness of one layer of piezoelectric material of the laminated piezoelectric element is 40 to 150 .mu.m.